Molecular mechanisms of nickel (Ni) genotoxicity were studied. The working hypothesis tested assumes that Ni derivatives initiate tumors through active oxygen species. We found increased contents of a DNA oxidation product, 8-hydroxy-2'-deoxyguanosine (8-OHdG) in DNA isolated from NRK-52 and NIH 3T3 cells and from kidneys of mice exposed to Ni(II). Moreover, the magnitude of 8-OH-dG increase by Ni was greatest in kidneys of BALB/c mice, i.e., mice which were also more susceptible to renal lipid peroxidation (LPO) by Ni than C3H, B6C3FI, and C57BL mice. No such concurrence was observed in NRK-52 and NIH 3T3 cells indicating that LPO and nucleobase oxidation might constitute two independent phenomena. Our hypothesis was also tested on pure 2'-deoxyguanosine (dG), DNA, and nuclear chromatin isolated from the human-derived K562/S cell line. Three major discoveries were made in these systems: (1) activation of hydrogen peroxide by Ni is facilitated by L-histidine (His), a principal in vivo Ni carrier, and by tetraglycine (TG), a model Ni-binding peptide; (2) Ni + H202 modifies all 4 DNA bases; besides 8-OH-dG, 10 more potentially mutagenic products were identified; (3) nuclear proteins enhance Ni-catalyzed attack of H202 or 02 on DNA. The relatively high sensitivity of guanine to attack by Ni-mediated oxygen radicals is consistent with the correspondingly strong complex formation between Ni and dG. It is also consistent with growing evidence that G:C base pairs are the major site of point mutation in the K-ras oncogene isolated from Ni-induced renal tumors in rats (see Project ZOICPO5399). Further, both TG and His enhanced DNA-protein cross-linking in kidneys of rats injected with Ni, a phenomenon characteristic for oxygen radical attack on cell nuclei. Our studies on the mechanisms of yet another genotoxic effect by Ni subsulfide (Ni3S2), deamination of 5-methyl2'deoxycytidine (5MedC), disclosed that the deamination is caused by 02 activated through autooxidation of the sulfur moiety of Ni3S2. Hence, sulfur is capable of increasing Ni genotoxicity. This may account for the uniquely high carcinogenicity of Ni3S2 compared with other Ni derivatives.